For increased fuel economy and lower exhaust emissions, it is common for automotive type engines to shut off the fuel during certain engine decelerations. The fuel shut-off occurs at an rpm higher than the normal engine idle speed. This sudden termination of positive engine combustion torque during deceleration results in an abrupt increase in the negative net engine torque which includes negative torques due to engine and vehicle friction, pumping losses, etc., so that a decided bump, or abrupt increase in vehicle deceleration may be experienced by the operator. The fuel shut-off is usually computer controlled to be operative as soon as it recognizes that the engine is in a closed throttle mode and that the vehicle speed, engine rpm and/or other conditions are right. The fuel shut-off, therefore, may occur at a point when the air mass flow rate through the idle speed control throttle bypass air valve is considerably greater than that required to establish the desired idle speed.
More specifically, where a throttle bypass air valve is included to control bypass air around the closed throttle valve, during normal open-throttle engine operation the bypass valve is typically controlled to allow an air mass flow rate that is larger than the rate required to maintain normal engine idle operation. The conventional closed throttle deceleration operation usually places the bypass valve in a dashpot action to smoothly decrease air mass flow rate to the engine to normally prevent stalling or unacceptable engine transient behavior. As a result, the fuel shut-off usually occurs at a point when the air mass flow rate is greater than the idle speed air mass flow rate. In particular, when fuel shut-off occurs, the air mass flow rate is never less than the rate required to maintain normal engine idle operation.
This invention provides a control operable during engine decelerations in response to predetermined operating conditions to delay the fuel shut-off until the total air mass flow rate to the engine is decreased below that level normally required to establish the engine idling speed, and immediately returned to the normal engine idle speed air mass flow rate following fuel shut-off, to provide a smooth engine deceleration without the conventional "bump" or abrupt increase in vehicle deceleration referred to above, and also to permit the normal idle speed condition to be reestablished upon subsequent restoration of fuel flow.